1. Field of the Invention
This invention relates to the controlled release of volatile materials. More particularly, this invention relates to dispensers for volatile liquid materials such as an air freshener fragrance or a biologically active composition. The dispenser consists essentially of a solid hydrogel containing 90 weight percent or more of a aqueous mixture containing the volatile material, which is dispensed at a uniform, controllable rate into the atmosphere adjacent to the dispenser, while the surface of the dispenser remains dry to the touch throughout the dispensing process.
2. Description of the Prior Art
Dispensers for the controlled release of volatile liquids such as perfumes, deodorants and biologically active compositions are known. One class of dispensers comprise a matrix, such as a film formed from a natural or synthetic organic polymer, that is impregnated with the material to be released. Dispensers of this type are described in U.S. Pat. No. 4,720,409, which issued on Jan. 19, 1988. One major disadvantage of this type of dispenser is the relatively small volume of liquid that can be incorporated into a solid film of an organic polymer without destroying the coherency of the film. This severely limits the useful life of the dispenser.
A second type of controlled release dispenser comprises a reservoir of the volatile liquid material enclosed within either a rigid or flexible container wherein at least a portion of the wall of the container is porous to the volatile liquid. Dispensers of this type are described in U.S. Pat. No. 4,356,969, which issued on Nov. 2, 1982. To reduce the likelihood of leakage of a bulk liquid from the dispenser, the liquid composition containing the volatile materials to be released can be absorbed into a porous material such as a sponge or a polyurethane foam. The impregnated material can optionally be enclosed within a porous container. Dispensers of this type are described in West German Patent No. 1,617,955, which issued on Apr. 22, 1971; French Patent No. 2,585,954, which issued on Feb. 13, 1987; U.S. Pat. No. 4,594,380, which issued on June 10, 1986; U.S. Pat. No. 4,094,119, which issued on June 13, 1978; French Patent No. 2,239,255, which issued on Apr. 4, 1975; and French Patent No. 2,229,425, which issued on Jan. 17, 1975.
In a third type of dispenser the liquid composition containing the material to be released is incorporated into a gel or wax formed from a natural or synthetic organic polymer. Additives to modify the properties of the gel of wax can also be included in the composition.
U.S. Pat. No. 4,117,110, which issued on Sept. 26, 1978, describes an improved solid air deodorant wherein the active ingredient, typically a perfume composition, is combined with paraffin that has been gelled using a metallic soap. The improvements alleged for this product are the presence of the soap, which acts as a support for the hydrocarbon gel, and the ability to select the volatility of the paraffin to achieve an evaporation of both the paraffin and the active ingredient, thereby leaving the external appearance of the dispenser unchanged.
U.S. Pat. No. 4,719,040, which issued on Jan. 12, 1988 teaches absorbing a perfume into a finely divided, water insoluble polymer and combining the resultant mixture with from 0.5 to 10 percent, based on the combined weight of all ingredients, of a gelling agent.
Published European patent application Ser. No. 138,844, which issued on May 2, 1985 teaches achieving continuous diffusion of perfume into the atmosphere by incorporating the perfume composition into a gelling agent such as carrageen or agar containing a mineral substance or fiber such as calcium sulfate, an aluminum silicate or sawdust.
In accordance with the teaching of British patent No. 1,544,221 a volatile material, such as an air freshener fragrance or an insecticide, is combined with a liquid hydrocarbon and a soap of a polyvalent metal, and the resultant composition is incorporated into a spongy gel formed from a lower alcohol and the sodium salt of a higher fatty acid such as stearic acid.
None of the aforementioned prior art gels is completely satisfactory as a dispenser for achieving controlled release of volatile liquids such as perfumes and biologically active compositions.
Two of the most objectionable disadvantages of many prior art gels as dispensers for the controlled release of volatile materials are a wet or otherwise aesthetically unattractive surface and/or the slow rate at which the volatile materials are released from the gel.
When many prior art gels are used to dispense a fragrance or other volatile liquid the gel must be enclosed in a suitable container that conceals the gel and makes it inaccessible to the user of the dispenser, yet allows the volatile liquid to be released into the atmosphere at the desired rate. When the volatile liquid is an air freshener fragrance it is often desirable to have a relatively high initial rate of release, in the order of about 1 gram per hour which then gradually decreases over a period of several hours to a lower rate that is maintained over the useful life of the dispenser.
The preparation of some prior art gels requires blending a number of ingredients in specified proportions to achieve the desired release rate. In other instances the desired release rate cannot be achieved and/or maintained over the entire useable life of the dispenser, or not all of the volatile liquid is capable of being dispensed.
Gels prepared from solutions of polyvinyl alcohol (PVA) in water or a mixture of water and a water-miscible organic solvent such as methanol, propanol, dimethyl sulfoxide and N,Ndimethylformamide are disclosed in numerous patents and other publications. Typical of recently issued patents describing gels formed from aqueous solutions of polyvinyl alcohol is U.S. Pat. No. 4,663,358, which issued to Hyon et al on May 5, 1987. This patent teaches cooling to below room temperature a solution of polyvinyl alcohol in a mixture of water and a water-miscible organic solvent such as alcohols containing from 1 to 4 carbon atoms, glycols and dimethyl sulfoxide. Mixtures of water and dimethyl sulfoxide are preferred, and the water can constitute from 10 to 90 weight percent of the solvent mixture. The gel formed by cooling the solution is then immersed in flowing water to remove the organic solvent. Alternatively the initial gel is dried to remove substantially all of the solvent and then immersed in water to form the hydrogel. This patent also teaches that extraction of the organic solvent may not be necessary for certain end-use applications of the gel, including the controlled release of drugs or perfumes.
The alleged advantage of hydrogels prepared as described in the Hyon patent is their transparency relative to the opaque gels obtained from solutions of polyvinyl alcohol in either water or dimethyl sulfoxide as the only solvent.
The Hyon patent teaches that hydrogels prepared in accordance with the method disclosed in this patent differ from hydrogels prepared at room temperature by the absence of stickiness and improved mechanical strength. The hydrogels are capable of being molded into articles of various shapes, however there is no indication regarding whether the aesthetic properties of the gel other than stickiness differ from those of other prior art gels, specifically, whether any liquid is initially present on the surface of the molded article or exudes to the surface when the article is exposed to the atmosphere for extended periods of time under ambient conditions. If the gel is to be used as a free standing dispenser for the controlled release of a perfume or other volatile material over a period of days or weeks, the presence of liquid materials during this period would be aesthetically unacceptable to the user of such a dispenser.
The present inventors discovered that controlled release dispensers prepared using PVA hydrogel compositions of the type described in the aforementioned Hyon et al. patent are unique with respect to dispensers formed from other hydrogels by virtue of surfaces that remain dry to the touch throughout the useful life of the dispenser. Unlike prior art dispensers, the dispensers of this invention are aesthetically attractive in addition to being free standing, and therefore do not have to be enclosed in a container to conceal the gal and/or prevent leakage of liquid materials that appear on the surface of the dispenser as the volatile material(s) entrapped within the dispenser is released. An advantage of the present dispensers is the ability to mold the gel from which the dispenser is prepared into any desired shape, including Christmas trees, other seasonal articles and various animals.
The present inventors also discovered an improved method for preparing dispensers from solubilizing polyvinyl alcohol that avoids the use of superatmospheric pressure.